1. Technical Field of the Invention
The present invention relates generally to an optical information reading apparatus. More particularly, the invention is directed to an improved bar code reading apparatus including a multifocal length optical system for finely focusing bar code images on an image sensor according to the distance between the apparatus and a bar code.
2. Background Art
Conventional optical bar code readers can be classified according to their optical information-reading modes as electronic scanning or laser scanning type devices. The electronic scanning type device reads a bar code in direct contact with a bar code label attached on an article, while the laser scanning type device indirectly reads the bar code at a distance therefrom. These types may be used appropriately according to the distance to the bar code. However, in recent years, there is an increasing need for a bar code reader which optically reads a bar code over a wide focal range between short and long distances to the bar code.
Japanese Patent First Publication No. 2-174362 teaches an improved bar code reader which includes an optical system with a variable diaphragm mechanism for assuring bar code reading in an extended focal range.
Such a prior art bar code reader, however, encounters the drawback in that when a lens opening is decreased for adjusting a focal length, it may cause the quantity of light reflected from the bar code to become lower than a sensitivity level of a photodetector.